1) RPG? 2) MMORPG?
I'm talking more about good content here.
Update: Because it is said here that volume of information is low in my question, and because there was too much to say, I created an extension question
Well Warhammer Online was made by somewhere between 300 and 400 people depending on who you ask and it is slowly tanking. Kingdom of Loathing has a dev team of about 4 or 5 and is awesome. Are either of those numbers useful? Not in the slightest.
100+ people for our project http://allods.com/ . It was really hard to gather so many high skilled people together in one place.
You don't have nearly enough information.
Is it 2D or 3D? How much character customization? How many creatures? How many textures? How much music/sound? How big is the world?
I could go on forever with thousands of questions you need to ask yourself before you can even attempt to figure out how many people it will take. Short answer is; anywhere from 1 to 1000 people.
We've made our MMORPG game in team of few friends, less than 10 people ( http://www.margonem.pl ): only 2 programmers, few artist and some content creators. But you need to consider 2 main things: time and cost. With good team, lot of money and time you can beat 10 times bigger teams with no funds. Smaller teams are also easier to maintain.
I think it depends on the scale of your MMORPG, and the level of graphical and control fidelity you expect. The smallest "AAA" MMO team I can remember was at EA and about 80 people, but it flopped. On the other hand, Puzzle Pirates succeeded (in its modest way) with about a dozen developers.
In general I suspect the number of backend engineers you need grows logarithmically with the number of expected users.
RuneScape Classic: two brothers (Paul and Andrew Gower) and a couple years. Very inspiring, if nothing else; of course, nowadays the current RuneScape game is managed by a large staff of which I could not find a number.